In recent years, methods for fabricating self-organized quantum wires and quantum dots have been actively researched. Self-organization represents a method of forming self-aligned three-dimensional structures such as a quantum wire or a quantum dot, utilizing mainly the mechanism of crystal growth. By employing quantum wires and quantum wells in active regions of devices, tremendous improvement of device properties, for example, a great reduction in threshold current in a semiconductor laser can be expected. In addition, self-organization attracts much attentions as a method of fabricating low damage and high quality quantum wires or quantum dots.
FIG. 10(a) is a cross-sectional view explaining a prior art method of fabricating a self-organized quantum wire, for example, reported by Koichi Inoue, et. al., Jpn. J. Appl. Phys. vol. 34 (1995), p. 1342. This diagram illustrates a section of a sample in which a GaAs/AlGaAs multilayer structure is grown by molecular beam epitaxy (MBE) on a GaAs substrate having an orientation of (110) inclined by 6.degree. or 3.degree. toward the 001! direction.
Generally, on such a vicinal GaAs substrate surface, atomic steps of two monolayers are formed at intervals in accordance with the inclination angle of the substrate. However, when the GaAs/AlGaAs multilayer structure is grown on this substrate by MBE, it is found that atomic steps are assembled with their growths to form multiatomic steps each having a terrace width of about 100 nm and a step height of 10 to 20 nm, as shown in the figure. This diagram illustrates a sample in which when an AlAs/Al.sub.0.2 Ga.sub.0.8 As/AlAs sandwich structure is formed after formation of five periods of GaAs/Al.sub.0.5 Ga.sub.0.5 As periodic structure, Al.sub.0.2 Ga.sub.0.8 As quantum wires are formed at the step edges. The quantum wire fabricated in the above manner has a characteristic that the light refractive index is lower at inclined regions and higher at flat regions of the step edges, and is applicable to an active layer of a laser or the like.
FIG. 11 is a cross-sectional view of quantum wire structures fabricated by another conventional method of fabricating quantum wires which is, for example, reported Shinjiroh Hara et al., Jpn. J. Appl. Phys. vol. 34 (1995) p. 4401. In this prior art example, the structure shown in the figure is grown on a substrate having a (001) surface inclined 5.degree. toward the 110! direction, by metalorganic chemical vapor deposition (hereinafter also referred to as MOCVD) to form quantum wires comprising an AlAs/GaAs/Al.sub.0.35 Ga.sub.0.65 As sandwich structure.
In either of the above two examples, multiatomic steps are formed using a vicinal substrate, and on the steps, quantum wires are self-organizingly formed by skillfully controlling slight differences in growth habit on the multiatomic steps between GaAs and AlGaAs.
The prior art methods for fabricating self-organized quantum wires described above have the following problems.
(1) For example, if GaAs growth employing a (110) vicinal substrate with the 001! direction as the inclination direction for the substrate is performed by MOCVD, multiatomic steps cannot be formed due to the difference in crystal growth habit between MBE and MOCVD, and surface roughness occurs, thereby making the fabrication of a quantum wire difficult.
(2) Since the growth conditions for which good quality multiatomic steps are formed by GaAs and AlGaAs have not been determined quantitatively yet, the stable formation of quantum wires is difficult.
(3) The growth in the crystal growth process is not sufficiently uniform for a minute structure like a quantum wire. Accordingly, a surface condition as shown in FIG. 10(b) occurs, making it difficult to form step edges with good linearity in the longitudinal direction of the quantum wire.